Transconductance-capacitance (Gm-C) filters offer attractive performance characteristics. Thus, the use of Gm-C filters is widespread and pervasive in radio communications and signal processing. Analog Gm-C filters are constructed using operational transconductance amplifiers (OTAs). OTAs operate to translate a voltage input signal into a current output signal. An example balanced (differential output) OTA is shown in FIG. 1a. The transconductance Gm for the OTA determines the I+ and I− currents based upon the input voltages V+ and V− according to the following equations:I+=Gm(V+−V−)I−=Gm(V−−V+)Various approaches are known to construct OTAs such as using cascodes or differential architectures. A simple analog transconductance-capacitance (Gm-C) filter may be constructed using a single-ended OTA as shown in FIG. 1b. If a time constant τ is defined as C1/Gm, then it can be shown that Vin for this filter equals Vout+τd(Vout(t)/dt. The cutoff frequency for the resulting Gm-C filter will thus rely on both Gm and C1. But process corner variations will typically be in the range of 20% for a desired capacitance whereas a desired transconductance will have process corner variations in the range of 10%. It follows that the resulting time constant τ for such a filter will be accurate to just 30% across all the process corner variations. Moreover, transconductance values will vary significantly with temperature and the supply voltage level. In addition, input noise will introduce variations in the filter coefficients. Accordingly, it is conventional to provide some sort of tuning circuitry on Gm-C filters. In this fashion, a tunable Gm-C filter has its time constant set to some desired value with some isolation from variations in the power supply voltage, process corner, and temperature.
Although such independence is desirable, conventional tunable Gm-C filters are still sensitive to power supply variations and suffer from non-idealities. Accordingly, there is a need in the art for improved tunable Gm-C filters that are more robust to variations in process corner, power supply, and temperature.